2.1 Context of the Invention
Wireless home audio and video applications are now increasingly numerous and require ever higher data bit rates of the order of some Gigabits per second (here below denoted as Gbps) and an increasingly higher quality of service. WPAN millimeter type home networks are particularly well suited to this type of application. Indeed, the authorized band about a carrier frequency of 60 GHz offers a wide bandwidth thus enabling the transportation of a large quantity of data. Besides, the radio range of such systems is limited to about ten meters, favoring the re-utilization of the frequencies in time and space.
The physical properties of the carrier bandwidth around 60 GHz and the regulatory ceiling on the power of the sender devices (or nodes) are currently limiting communications to a maximum of about ten meters.
Moreover, in this carrier frequency band, the attenuation of the radio signal in air is great.
In practice, in order to obtain high quality radio communications and sufficient radio range without needing to send at unauthorized power values, these characteristics make it necessary for the nodes of a home wireless network to have antennas configured directionally (or selectively) with high positive gain.
More particularly, antennas of this type, called “smart antennas” can be used to reach the distances required by audio and video applications within home networks. A “smart antenna” is constituted by an array of radiating elements distributed in a matrix on a given support. This network enables the implementing of a beamforming technique. In this technique, each radiating element of the antenna is electronically controlled in phase and power (or gain) to obtain a swiveling beam of varying narrowness when sending and/or receiving. The use of this type of antenna at reception of a radio signal increases the sensitivity of the antenna in reception in a desired direction and reduces the sensitivity of this antenna in areas of interference or highly noisy areas. The use of this type of antenna during the transmission of a radio signal increases the power of the radio signal in the desired direction.
In meshed communications networks, when a node sends out a radio signal (in sending mode), its smart antenna is adjusted to send out a wide radiating beam (wided angle of antenna radiation) so as to reach a maximum number of receiver nodes. When a node receives a radio signal (in reception), its smart antenna is regulated to receive data in a narrow and orientable antenna angle in order to increase the gain of the antenna and direct it to the node that has sent the radio signal.
In reception mode, each receiver node aims its reception antenna at an angle of orientation adapted to receiving data coming from the sender node. At each new sender node, each receiver node must therefore orient or aim its reception antenna according to a new angle of orientation adapted to the position of the new sender node in the network.
The search for an optimum angle of antenna orientation is a recurrent technical problem in antenna communications networks configured directionally. In a network where the exact positions of the devices are not known reliably, it is often necessary to carry out an exhaustive scanning of the coverage area of the antenna in reception mode in order to select an optimum angle of orientation for the receiving antenna for a communication link-up considered. The implementation of such an antenna scanning is especially necessary in WPAN home communications systems. Indeed, owing to the short wavelengths used (millimeter waves) such communications systems are highly sensitive to phenomena of interference and shadowing.
The drawback of such a method lies in the fact that it cannot be implemented during a communications link-up. Indeed, such a method requires the receiver nodes of the network to orient their receiver antenna and scrutinize in directions other than the one needed for receiving payload data sent out by a sender node considered, thus giving rise to a loss of payload data during this period of time.
The time division multiple access (TDMA) protocol is a protocol for a multiplexing mode used to transmit several signals on one and the same communications channel. This is time division multiplexing that relies on the principle of a division of the time domain into access sequences (more commonly called TDM network cycles) each access sequence into several time slots or speech times which are allotted successively to the different devices of the network. Each node of the network can therefore send out data in turn on a same radio communications channel, the other nodes being then either in a mode of operation for receiving data or in another mode of operation that does not disturb the radio communications channel, such as for example the standby mode.
The meshed communications systems (called mesh networks) implementing a TDMA protocol of this kind may rely classically on the presence of a master device responsible for setting up network connections, synchronizing the speech time of each of the nodes of the network and arbitrating on access to the shared wireless medium.
The meshed communications systems implementing transmission of data on the network according to a transmission redundancy mode are formed by a set of nodes communicating together through a plurality of communications paths. In mesh networks of this kind, a data content transmitted by a sender node may take different communications paths. In this way, a receiver node, which is an intended recipient of this data content, may therefore receive numerous copies of this same content of original data through different communications paths. This particular property is taken advantage of in certain communications systems to improve the reliability of the data received. For example, if a receiver node receives three copies of a same data content, this node may decide to select the faithful copies on the basis of the number of identical copies received.
Besides, the use of an error correction method is common in communications systems. Indeed, an error correction method for this kind makes it possible to obtain the maximum capacity of data transport from a transmission carrier. It furthermore makes it possible to characterize the error rate of the transmission channel and know its maximum value.
An error correction method of this kind can also include a mechanism to detect erasures (or missing symbols) by setting up correlations between each of the copies (of a same data content) received by a node that is the destination of the data content considered.
The redundancy of transmissions required for the meshing of a mesh communications network is implemented successively in time on a same radio communications channel. Indeed, when a sender node sends data on the radio communications channel, this data is received by the different nodes of the network, which are then playing the role of receiver nodes (i.e. working in reception node). These receiver nodes then re-send the data; preliminarily received from the sender node, taking turns according to the speech time slots allotted to them. Depending on the structure of the mesh of the network implemented, the data can be repeated from one to N times on the radio communications channel, N representing the number of nodes of the network.
Moreover, the implementing of a data transmission on a mesh communications network according to a transmission redundancy mode may prove to be particularly efficient in guaranteeing good reception beyond a predefined residual error rate. This technique is particularly well suited to applications requiring low bandwidth (of the order of about 10 Mbps) such as the transmission of audio data or control data for example.
However, because of a bandwidth that is generally limited for such home networks, the redundancy of transmissions applied in these mesh communications networks prove to be ill-adapted to applications calling for a data stream at a higher bit rate (of the order of about 100 Mbps) such as video applications for example. For this type of application, it thus proves to be more worthwhile to envisage only a single transmission of data on the network and implement an technique for adaptive routing of communications sensitive to the disturbances of the network, depending on the positions of the devices of the network and the sources of disturbance (shadowing, interference etc) preliminarily detected in the coverage area of the sending and reception antennas. Several alternative communications paths may therefore be used so as to enable an adaptation of the routing of the communications, the nodes of the network adapting the choice of the communications paths to the instantaneous disturbances of the network.
Communications paths of this kind can be set up in line-of-sight mode, namely aligned or in non-line-of-sight mode between a sender device and a receiver device of the network.
Here below, the term “aligned communication” or “line-of-sight communication” is understood to mean a communications link-up (or generally the setting up of a communications link-up) for which the receiver device parameterizes its antenna in reception so as to aim the direction of the sender device.
Here below, the term “non-line-of-sight communication” is understood to mean a communications link-up (or generally the setting up of a communications link-up) for which the receiver device parameterizes its antenna in reception so as to aim in a direction other than that of the sender device.
In the case of a non-aligned or non-line-of-sight communication, the routing of the data between the sender and receiver devices of the network through a communications path brings into action a relay node, which could be active, i.e. implementing a data relay adjusted in accordance with a predefined transmission protocol (authorizing a deferred relaying of the data received) or passive transmission, i.e. implementing a relaying of the data by reflection of the signal carrying the received data.
The devices working as active relays of the network are generally known to all the other devices of the network. These are in fact devices of the network having a particular function of data relays.
The passive relays for their part are not necessarily known to the devices of the network. This type of relay in fact does not have any means of communication with the other devices of the network because it does not have any sender or receiver antennas. It may be a wall or an object having reflective capacities such as a plate or a metal bar for example.
2.2 Technical Problem of the Invention
One of the main difficulties encountered when implementing a WPAN type home communications network based on a TDMA protocol and on directionally configured antennas is that of being able to swiftly and reliably determine the antenna orientation angles of a sender node and a receiver node so as to ensure constantly high quality communications of data in point-to-point transmission between these nodes.
The optimum aiming of an antenna in a given direction is generally difficult to maintain over time owing to the short radio wavelengths used in these wireless communications systems. Indeed, the smallest shift by the nodes of the network or a variation in the aiming precision due for example to a drift in temperature or an ageing of certain electronic components may affect the quality of a radio communications link-up.
This technical problem is particularly worrisome in the case of certain communications, especially involving data streams at high bit rates, which allow neither transmission redundancy nor a use of the bandwidth to carry out optimal aiming of the antennas of each node.
2.2 Prior-Art Approaches
There already exist various methods for adjusting antenna orientation angles in the prior art.
A known technique presented in the American patent document U.S. Pat. No. 6,498,939 (Texas instrument) proposes a method for adjusting the antenna orientation angles of a wireless communications system based on a measurement of the quality of the signal received by a receiver node through a first communications path (conveying applications data) and a transmission of control data via a second communications path independent of the first communications path.
One drawback of this prior art technique is that it makes it necessary to set up an additional communications path, namely a communications path other than the one used to transmit applications data, to obtain an adequate adjustment of the antenna orientation angles. Such a method therefore generates additional costs and complexity of implementation.
Another drawback of such a technique is that it cannot be used to ensure lossless transmission of application data, because the antenna adjustments cannot be done before each operation for sending application data, which is especially true in the specific case of transmission of data at high bit rates in a communications network supporting a highly fluctuating radio communications channel.